This type of conveyor has long been known for applications in which it is desired to locally decrease the speed of transport of articles without varying the drive speed of the chain.
U.S. Pat. No. 2,487,354 (Anchor Hocking Glass, 1949) describes a conveyor of glass containers intended to locally undergo heat treatment.
Said conveyor is equipped with a chain formed of links, said chain adopting a stretched configuration in a first narrow guide section, in which the links extend in line to form a single row of articles, and a compact configuration in a second wide guide section, in which the links are folded accordion-style to locally form two rows of articles.
In the wide section, the speed of travel of the containers is divided by two; the containers are exposed there to flames from gas jets providing the heat treatment (in this instance, polishing).
The chain circulates in a closed loop on two sprocket wheels, one of which, the drive wheel, is driven in rotation by an endless screw system engaging a pinion integral with the wheel.
The changeover of the chain from its stretched configuration to its compact configuration is accomplished simply by the pivoting of the links in the wide guide section, produced by the thrust of the following links, still located in the narrow guide section,
due to the local decrease in travel speed of the links entering the wide guide section.
Conversely, at the exit of the wide guide section, the links are pulled by the preceding links already located in the narrow guide section, and which have resumed a normal speed of travel.
In this type of conveyor, it has been noted that, beyond a certain threshold speed, instabilities appear in the pivoting movements of the links at the entrance to the wide guide section, due to the increasing amount of deceleration undergone by the links, which results in an increase in the thrust to which the links are subject. Because of their accordion-type disposition, the links exert increasing stress on the lateral surfaces of the wide guide section, in proportion to the thrust that they undergo. This stress generates increased wear of the contact surfaces, and can lead to a slowdown or even jamming of the chain in the wide guide section.
Consequently, this type of conveyor is only suitable for applications where the production rates remain low, typically the polishing of glass containers cited by the aforementioned document U.S. Pat. No. 2,487,354. Moreover, it will be observed that the endless screw drive system does not allow the drive wheel to be made to turn at high speeds.
However, this type of conveyor is not suitable for applications where production rates are high, typically in conveying plastic preforms for manufacturing containers, in which the rates can reach or exceed 50,000 units per hour.